Although the ultimate targets of many signal transduction pathways are nuclear actors, the vast majority of known protein kinases are cytosolic. This project investigates a novel human kinase that is present exclusively in the nucleus. Kinase activity is increased upon cellular proliferation and is markedly elevated in patients with acute and lymphocytic leukemias. We have identified a human gene that encodes this nuclear kinase and find that it is closely related to Drosophilia female sterile homeotic (fsh), a developmental regulator with no known biological activity. Collectively, these results suggest that this nuclear kinase is a component of a signal transduction pathway that plays a role in Drosophilia development and human growth control. The posttranslational modifications of the protein are nowbeing investigated by mass spectrometry. Later, the association of this protein with others in itsactive complex will be probed.